1. Field of Invention
The invention relates to silicon nitride compositions and, in particular, to silicon nitride bodies including lanthanum.
2. Discussion of Related Art
Silicon nitride (Si3N4) materials comprise a class of ceramics that has been shown to provide excellent characteristics in high wear applications including, for example, bearings, cutting tools, vanes and blades, and valves and seals. These properties include low density, high hardness, high temperature tolerance, high dielectric strength and extended wear capabilities. Typically, silicon nitride bearing elements have lower density, greater hardness, a higher elastic modulus and higher temperature ratings than do steel components. For these reasons and others silicon nitride components have replaced steel and alloy components in many applications. One example is the use of high speed hybrid bearings consisting of silicon nitride rolling elements and steel races.
Several methods of forming silicon nitride bodies are known. Typically, silicon nitride bodies are densified by applying heat and pressure to green bodies of silicon nitride powder. These techniques include sintering in the presence of a sintering aid and/or hot isostatic pressing (HIP) to form consistent durable bodies. Known sintering aids include, for example, aluminum oxide (alumina) and yttrium oxide (yttria). Sintering aids may be added in the form of a fine mesh powder to the silicon nitride powder prior to densification, and a variety of materials have been developed to improve characteristics, such as hardness.
U.S. Pat. No. 5,908,796 to Pujari et al. describes a silicon nitride body made using TiC of a small grain size. According to the Pujari disclosure, the resulting body provides improved wear and flexural properties. Products such as bearing components can benefit from these improved wear and flexural properties.